1. Field of the Invention
The present invention relates to a laminated substrate, a method of producing the laminated substrate, a nonreciprocal circuit element, and a communication device.
2. Description of the Related Art
Related art nonreciprocal circuit elements such as circulators, isolators, and so forth have characteristics such that they transmit electric power only in a predetermined direction and do not transmit an electric power in the transverse direction. For example, the isolators are used in mobile communication devices such as mobile telephones and portable telephones by utilization of the above-described characteristics.
As described in Japanese Unexamined Patent Application Publication No. 9-289402, for some of the above-described nonreciprocal circuit elements, desired electrical characteristics are obtained by trimming the capacitor electrodes of matching capacitors disposed on the surfaces of laminated substrates. In the nonreciprocal circuit elements, a matching capacitor is trimmed after the matching capacitor is connected to a center electrode. Since the capacitor electrode is covered with the center electrode, the area of a portion of the matching capacitor which can be trimmed is decreased, so that the range of the electrical characteristics can be adjusted becomes small. In particular, in recent years, nonreciprocal circuit elements for use in mobile communication devices have been further reduced in size, and the electrode areas of the matching capacitors have been further reduced in size. Thus, it is a problem that the areas of the capacitor electrode which can be trimmed become small. The trimming for adjustment of the electrical characteristic is carried out after the nonreciprocal circuit element is constructed. Therefore, if the trimming process fails, not only the laminated substrate but also the other elements such as a ferrite and a center electrode will be useless. The production cost of the nonreciprocal circuit element is thus increased.
Accordingly, to solve the above-described problems, a nonreciprocal circuit element (isolator) has been proposed which includes a laminated substrate 330 having matching capacitors C1 to C3 provided thereon as shown in FIG. 15, a ferrite having center electrodes disposed thereon to be mounted on the laminated substrate 330, a permanent magnet, and a metallic case covering these elements. In this nonreciprocal circuit element, the matching capacitors C1 to C3 are trimmed before the matching capacitors C1 to C3 are connected to the center electrodes.
More specifically, the laminated substrate 330 includes a dielectric sheet 342 having hot-side capacitor electrodes 371a to 373a, ground connecting electrodes 331, a circuit electrode 317, a resistor 375, and viaholes 318 formed in the surface thereof, a dielectric sheet 344 having hot-side capacitor electrodes 371a to 373b disposed on the surface thereof, and dielectric sheets 343 and 345 each having a ground electrode 374 disposed on the surface thereof. Moreover, a ground electrode to be soldered to the metallic case is disposed on the back surface of a dielectric sheet 345. While the laminated substrate 330 is in the state of a single member, trimming grooves are formed in the capacitor electrodes 371a, 372a, and 373a provided on the surface of the laminated substrate 330. The trimming is carried out in such a manner that the capacitor electrodes 371a to 373a are cut and separated. Accordingly, the above-described problems of the nonreciprocal circuit element according to Japanese Unexamined Patent Application Publication No. 9-289402 can be solved.
However, regarding the nonreciprocal circuit element having the laminated substrate 330 shown in FIG. 15, the trimming capacitor electrodes 371a to 373a are electrically connected to the center electrodes by use of a connecting material such as solder paste or electroconductive paste after the capacitor electrodes 371a to 373a are trimmed. This causes problems in that a part of the trimming grooves 380 may be filled with and embedded by the connecting material, so that the cut and separated trimming capacitor electrodes 371a to 373a are connected again. Especially, when the trimming is carried out so as to cut and separate the capacitor electrodes 371 to 373 by a laser, it is very probable that the trimming grooves are shortened, since the widths of the trimming grooves 380 are small, that is, are in the range of 10 μm to 100 μm. If the electrodes are partially cut, and the unnecessary portions are completely removed, a short circuit condition can be prevented. However, in this case, the trimming time is very long.
Moreover, the hot-side capacitor electrodes 371a to 373a are positioned near the ground connecting electrodes 331. Thus, it is very probable that they are short-circuited in addition to the trimming grooves 380.